Since magnetic north is slightly shifted from true north, true north cannot be measured using a magnetic compass. However, administrative maps are produced based on true north, and the Construction Standards Act is also based on true north. Therefore, in the field of civil engineering and construction, true north must be determined correctly. In underground tunnel construction, in particular, the magnetic compass does not function correctly because of the effect of a mineral vein.
There is a known conventional gyro compass that detects the earth's rotational angular velocity to determine true north. As disclosed by JP-A 2002-296037, U.S. Pat. Nos. 6,594,911 and 6,918,186, gyro compasses are generally a tri-axial orthogonal type device. These gyro compasses are large in size and costly to manufacture.
In order to reduce the size and cost, single axis or two-axis gyro compasses are suggested by JP-A 6-3149, JP-A 6-11350, JP-A 11-160072, JP-A 11-190633, and JP-A 2001-215121. Most of such gyro compasses have a gyro sensor and an acceleration sensor rotated on a rotation base. In these gyro compasses, however, a large space must be secured for its large rotation angle. This limits how compact the device can be. Most of single-axis gyro compasses need a horizontal plane, which makes it difficult to handle them. Single-axis gyro compasses that do not need a horizontal plane have been suggested, but their direction measuring accuracy is inferior to those of three-axis compasses.
Therefore, the applicant has suggested a single-axis type azimuth measuring device that does not need a horizontal plane in the disclosure of JP-A 2008-215956. The azimuth measuring device disclosed by the document takes into account a UVW rectangular coordinate system in addition to the XYZ rectangular coordinate system. Six directions, +U, −U, +V, −V, +W, and −W directions are provided apart at intervals of 60 deg when they are reflected orthogonally on a YZ plane. Elevation angles α formed between the U, V, and W axes and the YZ plane are each preferably from 30 to 40 deg, more preferably 35.26 deg. The reason for the arrangement is described in JP-A 2008-215956 or International Publication WO2010/047078 that will be described.
The azimuth measuring device includes a rotational angular velocity sensor that detects rotational angular velocities ωU, ωV, and ωW around the respective axes in the UVW rectangular coordinate system, a gravitational acceleration sensor that detects gravitational accelerations gU, gV, and gW in the respective axial directions, a first stepping motor that rotates the rotational angular velocity sensor and the gravitational acceleration sensor by 60×n° (n: a natural number) around the X axis for positioning, and a second stepping motor that swings the rotational angular velocity sensor and the gravitational acceleration sensor by ±35.26 deg around an axis orthogonal to the X axis for positioning. The azimuth measuring device measures the rotational angular velocities ωU, ωV, and ωW and the gravitational accelerations gU, gV and gW around the respective axes and subject the obtained rotational angular velocities ωU, ωV, and ωW and the gravitational accelerations gU, gV, and gW to coordinate transformation to produce rotational angular velocities ωX, ωY, and ωZ and gravitational accelerations gX, gY, and gZ in the XYZ rectangular coordinate system. Then, an azimuth angle ψ is calculated based on the obtained rotational angular velocities ωX, ωY, and ωZ and gravitational accelerations gX, gY, and gZ. Using the azimuth measuring device, the rotational angular velocity sensor and the gravitational acceleration sensor are swung only by ±35.26 deg, so that the rotation angle is small and a necessary space is not larger than the space required by the conventional azimuth measuring device. Therefore, the size can be reduced as compared to the conventional single-axis azimuth measuring device.
Furthermore, the applicant has proposed a six-direction directing device that can be directed in the above-described six directions, +U, −U, +V, −V, +W, and −W only with a single driving source in the disclosure of International Publication WO2010/047078. The disclosed six-direction directing device includes an inclined crank mechanism including a rotation member, a guide pin attached to the rotation member, and a guide that surrounds the rotation member. The guide has a case shaped and a track slit. The guide pin is inserted in the track slit and contacts with the track slit. When the inclined crank mechanism is rotated by the driving source, the guide pin rotates around along the track slit, so that the rotation member rotates. At the time, the rotational angular velocity sensor and the gravitational acceleration sensor are sequentially directed in the six directions, +U, −U, +V, −V, +W, and −W directions.